CAPF AC 2021 Q5: Science & Technology – Physics
- (a) A short-sighted person sees the near objects blurred but distant objects clearly and a convex lens is used to correct this defect.
- (b) A short-sighted person sees the near objects clearly but distant objects blurred and a concave lens is used to correct this defect.
- (c) A short-sighted person sees the near objects blurred but distant objects clearly and a concave lens is used to correct this defect.
- (d) A short-sighted person sees the near objects clearly but distant objects blurred and a convex lens is used to correct this defect.
Answer: (b) A short-sighted person sees the near objects clearly but distant objects blurred and a concave lens is used to correct this defect.
Explanation
Short-sightedness (myopia)
How to think: the name tells you the answer.
A short-sighted person has good sight only for short distances.
So near objects are clear and distant objects are blurred.
Then fix the lens: the eye bends light too much, so it needs a lens that spreads light, a concave lens.
| Feature | Myopia (short-sightedness) | Hypermetropia (long-sightedness) |
|---|---|---|
| Sees clearly | Near objects | Distant objects |
| Sees blurred | Distant objects | Near objects |
| Image forms | In front of the retina | Behind the retina |
| Cause | Eyeball too long or lens too curved | Eyeball too short or lens too flat |
| Corrected by | Concave (diverging) lens | Convex (converging) lens |
Answer: (b) near objects clearly, distant objects blurred, corrected by a concave lens.
Why each option is right or wrong
- (a) Incorrect. This describes hypermetropia (long-sightedness) and its convex lens.
- (b) CORRECT. In myopia near objects are clear, distant ones are blurred, and a concave lens corrects it.
- (c) Incorrect. The lens is right but the symptom is reversed; near objects are clear in myopia.
- (d) Incorrect. The symptom is right but the lens is wrong; a convex lens corrects hypermetropia.
Key point
Myopia: near clear, far blurred, image in front of the retina, concave lens. Hypermetropia: far clear, near blurred, image behind the retina, convex lens.
Remember
Myopia = minus = concave lens.
Common mistakes
- Reading short-sighted as someone who cannot see short distances.
- Swapping the lenses and giving a convex lens for myopia.
- Mixing up myopia with presbyopia, the weakening of the eye with age.
Exam tip
Educated guess: short-sighted means sight works at short range. That leaves (b) and (d). Myopia has a minus power, and minus power means concave.
Sources
- NCERT Class X Science — The Human Eye and the Colourful World (defects of vision)
- NCERT Exemplar Class X Science — The Human Eye and the Colourful World